The present invention relates to a rotation detector or more in particular to an electrode construction of an incremental encoder rotational position detector of electrostatic capacity type, which is capable of detecting the rotational speed of a rotary object such as an engine crank shaft of the automobile without delay regardless of the speed of the rotary object.
A conventional rotational position detector of electrostatic capacitance type comprises: a first plate including a first electrode having a plurality of electrode pieces radially arranged at equal spatial intervals and electrode pieces arranged radially between the electrode pieces of the first electrode; a third electrode and a fourth electrode; a second plate including 5th, 6th, 7th and 8th electrodes arranged opposedly to the first, second, third and fourth electrodes of the first plate respectively, the 5th electrode of the second plate being electrically connected to the 7th electrode thereof, the 6th electrode of the second plate being electrically connected to the 8th electrode thereof; means of supplying signals of opposite phases to the first and second electrodes of the first plate or the third and fourth electrodes thereof; and means of detecting the rotation of the second plate by detecting the change of the electrostatic capacitance generated between the first and second plates when the second plate is rotated in relation with the first plate.
The above-mentioned conventional electrode structure has such a disadvantage that among the signals of opposite phases applied to the first and second electrodes or the third and fourth electrodes of the first plate, the distributed capacitance formed mainly by the first and third electrodes causes the signal applied to the first or third electrode to be transmitted undesirably to the third electrode or the first electrode respectively, thus making it impossible to detect the signal satisfactorily, that is, to detect the rotation satisfactorily.